Beam deflectors used in laser scanners and printing systems cause cross axis spot placement errors due to mechanical wobble of the deflecting mirror in oscillating mirror scanners and due to facet to facet and bearing noise errors in spinners. Small errors become objectionable for high resolution printing and scanning systems. The usual techniques for measuring cross axis wobble and scan amplitude stability are laborious and the data obtained is difficult to interpret. A method based on real time signal analysis is presented which quantifies the scanner performance quickly and accurately.